It is a commonplace procedure to determine a material property by detecting measuring and reference radiations that have been emitted, absorbed, transmitted or reflected by the material. The terms "measuring" radiation and "reference" radiation are used herein simply to distinguish between two radiations that interact with the material in a different way. One radiation may interact selectively with the material, while the other radiation may exhibit only a generalized interaction.
Some examples of selective interaction include molecular resonance absorption in the infrared region, color filtration in the visible region, and K-edge absorption in the X-ray region of the electromagnetic spectrum. Some examples of generalized, non-selective interactions include the absorption of infrared radiations having wavelengths that are spectrally removed from the absorption bands of the material being measured, the filtration of light with neutral-density filters, and the absorption of beta rays by various materials.
The moisture content of a continuously-produced sheet of paper is commonly inferred from measurements of infrared radiation that is generated by an infrared radiation source means on one side of the sheet, directed to pass through the paper, and detected on the opposite side of the sheet. The detected radiation is separated into two narrow bands of wavelengths, including a band around 1.9.mu. that is herein termed a measuring radiation and a band around 1.8.mu. or 1.7.mu. that is herein termed a reference radiation. If it is desired to also measure the basis weight of the paper with the same instrument, an additional band of measuring radiation may also be separately detected as described in U.S. Pat. No. 3,405,268.
For various measurements, additional bands of reference radiations may be needed to correct for the effects of certain wavelength interactions with the microstructures of the material, and special wavelengths may be used to correct for the presence of certain ingredients or surface effects that exist in the material, for example, as described in U.S. Pat. No. 4,085,326.
It has generally been preferred to time-multiplex the measuring and reference radiations for alternate or sequential detection by a single detector in a single information channel, and to effectively compute the ratio M/R of the information channel response. This permits common-mode cancellation of many extraneous variables that influence the detected values of either radiation by itself. More-over, the apparently inevitable drifts that occur in the detector response can be made to appear as simple gain changes in the single information channel, and likewise cancel when the ratio is taken. However, such factors as shifts in the spectral content of the infrared radiation source, spectral shifts in one or both filter passbands, certain kinds of dirt accumulation on the windows over the source and detector, and certain electronic drifts do not admit of common-mode cancellation, and hence a standardization procedure is also required.
Standardization is commonly performed in a manner consistent with the law of Bouguer (sometimes referred to as the Lambert-Beer law of absorption) EQU I=I.sub.o e.sup.-.mu.x
in the form ##EQU1## Here I.sub.o represents the original intensity of radiation, and I represents its intensity after passing through a material having a mass per unit area x and an absorption coefficient .mu.. The intensities I and I.sub.o are usually derived as voltages V and V.sub.o respectively, from the information channel containing the detector. Hence, equation (1) can be expressed as ##EQU2## In the case of a dual-wavelength instrument, two such relationships are obtained ##EQU3## where the subscript M indicates that the voltage or absorption coefficient is associated with the measuring radiation and the subscript R indicates that the voltage or absorption coefficient is associated with the reference radiation.
Taking the ratio of the measuring and reference voltage responses results in a combined relationship ##EQU4## when x=o, ##EQU5## The condition x=o is achieved, or simulated, by removing the material from between the source and detector, or by moving the source and detector to an off-sheet position, or by inserting a standardizing flag between the material and the source and detector, as variously described in U.S. Pat. Nos. 2,829,268, 2,951,161 and 3,803,414 and in 4,085,326 supra. The detector then receives, from the source, radiation that has not interacted with the material, and the instrument is enabled to restore its calibrated accuracy.
The standardization procedure ensures that suitably current and updated values for the quantities V.sub.Mo and V.sub.Ro (voltages or other forms of response values representing the original intensities of the measuring and reference radiations from the source) are available. These values may be placed in relatively long term storage (between standardization periods), for example, in a computer memory. They may then be used each time a material property (x) value is to be computed from the current measuring and reference response voltages V.sub.M and V.sub.R obtained while the material is being measured. Alternately the V.sub.Mo value can be set equal to the V.sub.Ro value on standardization, and their proper mutual relationship maintained, by long-term storage of a gain factor, as described, for example, in U.S. Pat. No. 4,085,326 supra.
In some cases the values V.sub.Ro and V.sub.Mo may be derived in a simulated fashion by using a radiation chopper to direct the radiation along a reference path, alternately with its direction along the measuring path, and utilizing short-term storage of the signals during portions of the revolution time of the chopper, as described, for example, in U.S. Pat. Nos. 3,957,372 and 4,097,743. This allows some compensation where certain errors exist and can become serious enough to need correction in a time period shorter than the normal standardization interval. However, it can also introduce new sources of error, due to possible extraneous variables associated with the reference path, that would not otherwise be encountered.
In the single-detector, dual-wavelength multiplex system, the detector "sees" the material being measured, first as it is illuminated by the reference radiation, next as it is illuminated by the measuring radiation, then again as illuminated by the reference radiation, and so on in a continuous, alternating series. When the instrument is used to measure a fast-traveling material, the material moves an appreciable distance in the time required for the switch from reference radiation to measuring radiation and from measuring radiation to reference radiation. Hence, the detector sees a series of spots on the material, with alternate spots actually or effectively illuminated by the reference radiation, and with the other spots actually or effectively illuminated by the measuring radiation. These spots may overlap to a greater or lesser extent, or they may not overlap at all, depending on the rate of movement of the material relative to the rate and manner of switching back and forth between measuring and reference radiations.
The responses of the information channel (containing the detector) to the measuring radiation-illuminated spots are separately averaged, or one or both are placed in relatively short-term storage, for comparison, usually by effectively taking the ratio M/R of the measuring and reference response values. The fact that the spots effectively illuminated by the measuring and reference radiations generally exhibit at least slightly different values of the property being measured (or of an interfering property) is not of any consequence ordinarily. Time averaging of the channel responses is used and chopping of the radiations from the source is generally necessary in any case involving radiations such as infrared in order to avoid the effects of changing ambient radiation.
There are, however, situations where special provisions have been necessary to avoid substantial errors that could otherwise arise from the use of the "two-spot" or plural spot method of measurement. The thickness of a polyethylene coating on paper is commonly measured by detecting reference radiation and measuring radiation reflected from the interface between the paper and the polyethylene. However, in one case the paper was imprinted with printing ink, applied in a repetitive pattern along the length of the traveling sheet, before the polyethylene coating was applied. The printing ink was an effective absorber for the infrared radiation wavelengths used. It was found that at certain line speeds particular phase relationships developed between the moving pattern of printing on the paper and the alternation of the reference and measuring wavelengths received by the detector, causing the polyethylene thickness measurements to substantially deteriorate.
To correct the measurements, an auxiliary band of wavelengths (e.g., around 1.4.mu.), herein termed a "formation-monitoring radiation" was used together with an auxiliary second radiation detector that is herein termed a "formation detector". Now when a spot of reference radiation was directed into the material, the spot on the material was simultaneously illuminated with the formation-monitoring radiation. Similarly when the spot of measuring radiation was directed into the material, this spot on the material was likewise simultaneously illuminated also with the formation-monitoring radiation.
The principal radiation detector was equipped with a filter that substantially prevented the formation-monitoring radiation from reaching it. However, the principal detector still received the reference and measuring radiations alternately, and hence functioned in the same manner as before. The auxiliary formation detector was equipped with a filter that pevented the reference radiation and the measuring radiation from reaching it. However, when the principal detector received a pulse of reference radiation from the material, the formation detector received a pulse of formation-monitoring radiation, at the same instant and from the same illuminated spot on the material. Likewise, when the principal detector received a pulse of measuring radiation from the material, the formation detector received a pulse of formation-monitoring radiation, at the same instant and from the same spot on the material that provided the measuring radiation pulse.
The auxiliary information channel containing the formation detector provided a response F.sub.M to the formation-monitoring radiation arriving simultaneously with the arrival of the measuring radiation at the principal detector. The formation detector also provided a response F.sub.R to the formation-monitoring radiation arriving simultaneously with the arrival of the reference radiation at the principal detector. Hence the ratio F.sub.M /F.sub.R provided a measure of the degree of change that took place in the interfering property (the amount and kind of printing ink present to absorb the measuring and reference radiations) during the time interval between the detection of the measuring and reference radiation pulses from the material. The ratio F.sub.M /F.sub.R could then be used to correct the ratio R/M of the reference and measuring responses in accordance with ##EQU6## or in accordance with the analog computation actually used ##EQU7## in a manner somewhat similar to that described in U.S. Pat. No. 4,085,326 supra.
It is also a commonplace procedure to detect the measuring radiation and the reference radiation with separate detectors, in separate information channels. In a few cases this is essential because the characteristics of the radiations, and the necessary structures of the detectors, are markedly different. A system using two detectors in this manner has been perceived to have a theoretical advantage, in that it provides a "single spot" measurement, at least to the extent that the measuring radiation pulses and the reference radiation pulses can be detected at the same instant in time as well as separately. This system also should have no difficulties with the printing ink application just described. In applications such as the measurement of moisture in heavy, highly non-uniform paper board, for example, using infrared radiation, it should reduce the effects of non-linear averaging and noise, thereby reducing the averaging time required for measurements to a desired degree of resolution.
However, the detectors used in such applications, and other elements of the information channels, are subject to short-term drifts and instabilities that make them respond differently, at different times and in an unpredictable way, to corresponding radiation stimuli. When one or both channels exhibit response changes or response components that have no counterpart in the response of the other channel, there is no common-mode cancellation of their extrinsic effects on the measurement, as is the case with the dual-wavelength, single-detector system. These problems have so far prevented many of the most attractive theoretical benefits of the dual-detector, separate measuring and reference channel systems from being realized.
Where these systems have been used, in addition to the usual standardization provisions, some rather complex arrangements have been made to slow down and/or minimize the rate of drift in the detectors, by cryogenic gas or thermoelectric cooling, close temperature regulation of the detectors and other information-channel components to a tenth of a degree or so, and prevention of sudden and/or large changes in the intensity of the radiation falling on the detectors. More frequent standardization has been used to prevent the errors that do occur from becoming too large before they are corrected, say, each time the detectors are scanned across the width of a traveling paper sheet.
Additional steps in the standardization procedure have been added, including the insertion of internal standard radiation absorbers (filters) both to simulate the effect of a predetermined change in the basis weight (mass per unit area) of the sheet and to simulate a predetermined change in its moisture content. These standards may allow the extent of the existing errors to be computed or mathematically stated, and the instrument readings corrected, in a manner such as those described, for example, in U.S. Pat. Nos. 3,851,175 and 4,006,358. The effectiveness of such techniques do depend, however, on how well the constancy of the filters, and the precision of the mechanical filter insertion mechanisms, can be maintained. This can be very difficult in the hostile environments to which many industrial instruments must necessarily be subjected.
As shown by U.S. Pat. No. 4,057,734, it is known to use a spectrometric instrument for detecting gases such as sulfur dioxide in the atmosphere by measuring the spectra of light reflected from the earth that has traversed the atmosphere. A beam of this light is split by a beam splitter into two parts that are passed through respective measuring and reference wavelength band pass filters to respective measuring and reference radiation detectors in two separate information channels. The instrument contains an internal radiation source for producing a second beam of radiation that is herein termed channel-monitoring radiation.
A radiation chopper is arranged to periodically substitute the internally generated second beam for the first beam whose spectra are being measured. During the substitution periods the measuring radiation wavelengths contained in the channel-monitoring radiation beams are passed to the measuring detector, and the reference wavelengths contained in the channel-monitoring radiation beam are passed to the reference detector. At this time an automatic gain control (AGC) loop adjusts the gain in the measuring information channel to make its output equal to that from the reference channel. In another position of the chopper, it blocks both beams, and another control loop adds or subtracts a d.c. voltage in the measuring channel so that the dark current signal levels are equal in the two channels. In still another position of the chopper, the channel-monitoring beam of radiation is passed through a filter, such as a cell containing a sample of the gas that is being measured. The difference between the signals obtained from the two channels at this time is indicative of the sensitivity of the spectrometer, and may be used to automatically maintain the sensitivity at a constant level.
It was suggested that instead of using AGC feedback loops the outputs of the photodetectors could be converted to digital signals and the unbalance of the detectors could be compensated using conventional digital computing techniques. The instrument could be used to measure phenols in a water supply using the ultraviolet absorption bands of phenols in water. It could be used to measure gases present in a smoke stack by the absorption of a beam of light passed through the stack. To measure the fluorescence of materials in sunlight, or for sensing gases using the reflected solar spectrum, the internal channel-monitoring radiation source is replaced by a light pipe of fiber optics to introduce light from the sky into the spectrometer. In the latter two embodiments, the radiations from the measuring and reference radiation source (in this case, the sun) are passed to the detector through two paths alternately, the first path being the path through the material to be measured and the second being a reference path, in an arrangement similar to that of U.S. Pat. Nos. 3,957,372 and 4,097,743 supra.